Linear predictive coding (LPC) is extensively used in digital speech transmission, automatic speech recognition, and speech synthesis. One such digital speech coding system is disclosed in U.S. Pat. No. 3,624,302 issued to B. S. Atal, Nov. 30, 1971. The arrangement therein includes a linear prediction analysis of input speech in which the speech is partitioned into successive time frame intervals of 5 to 20 milliseconds duration, and a set of parameters representative of the time interval speech is generated. The parameter signal set includes linear prediction coefficient signals representative of the spectral envelope of the speech in the time interval, and pitch and voicing signals corresponding to the speech excitation. These parameter signals are encoded at a much lower bit rate than the speech signal waveform itself and a replica of the input speech signal is formed from the parameter signal codes by synthesis. The synthesizer arrangement comprises a model of the vocal tract in which the excitation pulses of each successive interval are modified by the interval spectral envelope prediction coefficients in an all pole predictive filter.
One well known method for generating speech feature signals involves speech analysis in which the autocorrelation of a time frame portion of a speech pattern are formed. The autocorrelation signals are then processed in accordance with the technique known as Durbin's recursion to generate signals that correspond to LPC coefficients, reflection coefficients, and the prediction residual energy of the time frame interval. While Durbin's recursion signal processing may be readily implemented in large general purpose computers, it is particularly useful to perform these processing operations in a single programmable digital signal processor (DSP) integrated circuit so that the processing equipment is small and economical. As is commonly known, however, the storage capacity in available DSP devices is generally small, and the DSP memory addressing capabilities are severely limited.
Transformation of an autocorrelation vector signal to a representation based on prior art linear prediction coding by the method of Durbin's recursion requires that operands be accessed from three single-dimension vectors and a two-dimension array. These requirements generally exceed the limited arithmetic addressing capability of a typical digital signal processor. As a result, it is necessary to store the signal processing instructions for each iteration of Durbin's recursion separately. Thus, a distinct set of instruction code signals is required for each iteration processing and the distinct sets are stored separately in the control memory of the digital signal processor. This stringing of the separate iteration instruction codes uses a large portion of the program memory and limits the utility of the DSP for speech processing applications. If all iterations required for Durbin's recursion could be performed by a single set of instruction code signals, processing of all iterations could be done by transferring control to a single subroutine occupying a predetermined number of control memory locations whereby DSP speech processing is rendered more efficient and more economical. It is an object of the invention to provide improved digital speech signal processing in real time digital signal processors.